The present invention relates to a vehicle steering system which can perform an automatic steering control and cancel the control.
There are proposed techniques for performing an automatic steering of a vehicle using a steering actuator provided on the vehicle. For example, a power steering system installed on a vehicle includes an electric motor or a hydraulic actuator as a steering actuator for imparting a steering force (a steering assist force) to a steering mechanism. Then, steered road wheels of the vehicle can be turned by controlling the output of the steering actuator without involving the manipulation of a steering wheel by the driver.
More specifically, in an automatic parking assist control, a target traveling path from a current position to a parking position of the vehicle is calculated, and a relationship of a target turning angle with a traveling distance of the vehicle is obtained so that the vehicle is reversed along the target traveling path so calculated. Then, the steering actuator is controlled based on a steering angle command value which is determined so as to attain a target turning angle according to a traveling distance of the vehicle. As this occurs, the driver only has to perform the adjustment of stopping position as well as the adjustment of vehicle speed by controlling the brake and accelerator pedals and does not have to manipulate the steering wheel.
When the steered road wheel strikes an obstacle such as a curbstone while the automatic parking control is in operation, since the steering actuator cannot steer the steering mechanism, an actual steering angle cannot follow a steering angle command value. In addition to this, in the case of an electric motor being used as the steering actuator, there is generated a state in which a maximum current continues to be supplied to the electric motor, the electric motor being thereby put in an overheat state. Then, in a related art technique disclosed in JP-A-2004-249913, it is determined that a steering disablement state occurs in response to the occurrence of a state in which a steering angle detection value does not reach the steering angle command value and an accumulated time period during which a motor current value is large has reached a predetermined determination time period, thereby the automatic parking control is cancelled in the midst thereof.
For example, when the vehicle is parallel parked in a small space by the automatic parking control, there may occur a case where a “steering-without-driving” in which the steering mechanism is driven in such a state that the vehicle is being stopped is necessary. However, there may occur a case where a long time is necessary until the actual steering angle comes to coincide with the target steering angle when the steering-without-driving is performed due to a shortage of output of the steering actuator. As this occurs, since a state in which a deviation between the actual steering angle and the target steering angle is large continues, in the related art technique, the automatic parking control is cancelled.
On the other hand, JP-A-2001-1930 discloses an automatic steering system in which an automatic steering control is cancelled when a steering-without-driving is performed in order to prevent an overload from being applied to a steering actuator. However, if the steering-without-driving by the automatic steering control cannot be performed, an automatic steering function is easily cancelled in the case the automatic steering control is desired especially at the time of parallel parking of the vehicle.
Thus, in the conventional automatic steering functions, there is a tendency that the automatic steering control is easily cancelled when the steering-without-driving is performed, and those conventional automatic steering functions have not been easy to handle.